A common issue among BMW E90 owners is experiencing slow starts, often accompanied by a low resting voltage. This article explores the relationship between resting voltage and starting performance in a BMW E90, using real-world data and observations to diagnose potential problems.
Investigating Low Resting Voltage and Slow Starts in a BMW E90
A recent experiment documented the resting voltage and starting performance of a BMW E90 over several days. The initial observation revealed a resting voltage of 11.89 volts after the car sat for two days, considerably lower than the expected voltage. Despite the low voltage, the car started, albeit with the voltage dropping to 8.02 volts during cranking.
After a short drive, the resting voltage recovered to a stable 12.67 volts, indicating the charging system functioned correctly. Subsequent starts showed voltage drops to 9.44 volts, then 9.01 volts, and finally a significant drop to 7.45 volts.
Comparing the voltage drop during these starts, even the “normal” start exhibited a prolonged cranking period, though with a higher voltage. Notably, the resting voltage before each start was similar, ruling out extremely low pre-start voltage as the sole cause of the slow starts.
Following these incidents, the resting voltage was 12.28 volts, and subsequent starts showed voltage drops in the 9-volt range. While an ammeter reading would provide more insight into the starter’s current draw, the available data points towards a potential battery issue.
Battery Health and its Impact on E90 Starting Performance
Battery analysis revealed an internal resistance of 7 milliohms and a State of Health (SOH) of around 21%, according to a battery analyzer. The car’s Intelligent Battery Sensor (IBS) reported a slightly higher SOH of 33%. The battery, being 31 months old, is nearing the end of its typical lifespan. A new replacement battery of the same model exhibited an internal resistance of only 3 milliohms.
The observed rapid self-discharge or insufficient charging by the IBS raises further concerns. Using a 2-amp maintenance charger overnight eliminated the slow start issue for 10 days, suggesting a charging deficiency. However, the significant variation in voltage drop between starts remains puzzling.
Potential Causes and Solutions for Low BMW E90 Resting Voltage
While a voltage drop below 8 volts during starting often indicates a short circuit, the consistent starting success despite the low voltage suggests other possibilities. A failing battery, despite showing a decent SOH according to the IBS, remains the primary suspect. The high internal resistance and successful mitigation of the issue with external charging support this theory.
Prolonged use of a failing battery can damage the starter, while a faulty starter can damage a new battery. Continuing to use the existing battery with weekly charging might provide a temporary solution, but installing the new replacement battery offers a more reliable long-term fix. This should prevent further slow start issues and protect the starter from potential damage.
Ultimately, replacing the aging battery is the recommended course of action to ensure reliable starting and prevent potential damage to other components. While a parasitic drain or starter issue cannot be definitively ruled out without further testing, the evidence strongly suggests the battery is the root cause of the observed symptoms.
